Separating and removing impurities, particularly nitrogen, included in natural gas is an important challenge for efficient utilization of methane, a main component of natural gas, as a fuel in fuel and chemical industries. With a recent shale gas boom, interests in separating nitrogen and methane have grown, and an energy value per unit volume of natural gas is reduced depending on the content of nitrogen included in liquefied natural gas, that is, natural gas in LNG preparation, and therefore, selectively removing nitrogen therefrom is an important matter affecting the value of natural gas. As one example, 25% of US natural gas contains rather high nitrogen content, and this becomes a limitation in directly using the natural gas as a raw material. With shale gas, a technology preparing high concentration methane by selectively separating and removing nitrogen included in a 5% to 20% range has recently been magnified. Specification of natural gas actually transferred to a pipeline is defined so that nitrogen content does not generally exceed 4% in order to satisfy calories, and accordingly, a process of removing nitrogen from natural gas prior to transfer is essential in order to follow the corresponding regulation.
Nitrogen included in natural gas is largely divided into two types, and one is nitrogen included in a natural gas field, and the other is a case of nitrogen introduced in large quantities for an enhanced oil recovery (EOR) process being mixed with natural gas. Nitrogen content included in a natural gas field is very different depending on a natural gas field, and varies from minimum 0.2% to maximum 25%. Among these, natural gas collected from a natural gas field containing 5% or more of nitrogen requires a nitrogen removing process called a nitrogen rejection unit (NRU), and collecting methane released together with nitrogen by the NRU process is an important issue. In the EOR process, 4% to 75% of nitrogen is included in collected hydrocarbon gas, typically methane, and removing nitrogen therefrom and collecting methane is also an important issue.
In order to separate large quantities of nitrogen, a cryogenic distillation method using a boiling point difference between nitrogen and methane has been widely used in the art. However, this method has a problem in that a boiling point difference between the two gases is only approximately 34° C. with the boiling point of nitrogen being −196° C. and the boiling point of methane being −162° C., and energy costs are excessively high since the gases need to be separated after being cooled to −162° C. or lower and distilled. Particularly, in the case of a small-scale gas field, using a process requiring excessive investment costs such as a cryogenic distillation method in order to remove nitrogen from natural gas is not economical. Accordingly, as an alternative separation method, a pressure swing adsorption (PSA) separation method, a temperature swing adsorption (TSA) separation method, a separation method using simulated moving bed (SMB) separation and a separation membrane, or a hybrid separation method combining PSA and membrane separation using a nitrogen selective adsorbent has been proposed. In order to complete such technologies, development of an adsorbent having high selectivity for nitrogen and excellent adsorptivity is the key. A PSA separation method is currently used to separate nitrogen and methane in a small-scale process, however, performances of existing commercial adsorbents are not outstanding enough to overcome the separation technology using a cryogenic distillation method, therefore, continuous development of technologies is required.
As the commercial adsorbent known to be effective in separating nitrogen and methane, a titanium silicate-series microporous Zeolite molecular sieves named ETS-4 is known. Several other adsorbents have been reported, but are insufficient in the performances to be used in a commercial separation process, and ETS-4 is considered as almost the only one adsorbent for separating nitrogen/methane. ETS-4 was developed by Engelhard Corporation (currently merged to BASF), a US catalyst company, and disclosed in the U.S. Pat. Nos. 4,835,202 and 4,928,929 in 1989 and 1990, and a technology separating nitrogen-containing methane using Ba-ion exchanged ETS-4 Zeolite was reported in the U.S. Pat. No. 5,989,316 in 1999. Furthermore, Sr-ion exchanged ETS-4 was used in separating nitrogen/methane, nitrogen/oxygen and argon/oxygen, and successful adsorption and separation was reported in 2001 (Nature, 412: 720-724 (2001)). The concept used herein is a “molecular gate” or “molecular sieve” effect, and by controlling pore sizes through changing a dehydration temperature of the Sr-ion exchanged ETS-4, it was used in adsorbing and separating nitrogen, methane, oxygen and argon having similar sizes in a range of 3 Å to 4 Å. The adsorbent exhibits a selective separation property for nitrogen so as to distinguish molecular sizes, however, there is a basic problem in that an adsorption amount for nitrogen is not high since adsorption rate differences induced from the molecular size differences are used in separation. In order to enhance separation efficiency represented by gas productivity selected in a PSA method, having high adsorption capacity for nitrogen and a high adsorption rate are preferable. However, according to the disclosure in the U.S. Pat. No. 5,989,316, adsorption capacity of Ba-ion exchanged ETS-4 Zeolite for nitrogen is approximately 9 ml/g at 1 atmosphere and 25° C., and it is seen that the Ba-ion exchanged ETS-4 Zeolite exhibits an advantageous property for nitrogen only in terms of an adsorption rate, and does not exhibit an excellent equilibrium adsorption amount. Accordingly, discovery of a nitrogen selective adsorbent having not only a high adsorption rate but also an excellent equilibrium adsorption amount has been required.
Meanwhile, air is a gas mixture including approximately 78% of nitrogen, 21% of oxygen and less than 1% of argon, carbon dioxide and the like, and is most common gas on earth. However, oxygen and nitrogen are widely used in the form of gas or liquid for industrial, research or medical purposes, and accordingly, studies on a method separating and concentrating these gases are required. In order to separate nitrogen or oxygen from the air with high purity, a method of cryogenic distillation using differences in boiling points of each gas has been used in the art. However, nitrogen has the boiling point of −196° C. and oxygen has the boiling point of −185° C., which is only a 11° C. difference in the boiling points of the two gases, and there is a disadvantage in that energy consumption is high since the gases need to be separated after being cooled to −185° C. or lower and distilled. In addition to the above-mentioned method, a method of using a commercial adsorbent having selective adsorptivity for nitrogen compared to oxygen such as Li-ion exchanged X-type Zeolite, so-called LSX Zeolite, a PSA method using differences in adsorptivity depending on a pressure, a separation method using a polymer separation membrane, or the like is used for air separation. However, these methods require a strict condition such that a special separation condition is required, and has high energy consumption, and therefore, are faced with the task of further enhancing energy efficiency.
An organic-inorganic hybrid nanoporous material, so-called a metal-organic framework, is also generally referred to as a “porous coordination polymers” or as a “porous organic-inorganic hybrid”. The metal-organic framework has recently begun to be newly developed by grafting molecular coordinate bond and material science, and the metal-organic framework has been actively studied recently since it is not only applicable to an adsorbent, a gas storage material, a sensor, a membrane, a functional thin membrane, a drug-delivery material, a catalyst, a catalyst carrier and the like, but also is capable of being used in separating molecules depending on molecular sizes by collecting guest molecules smaller than pore sizes or using the pores, since the framework has a high surface area and molecular-sized or nano-sized pores. In addition, the metal-organic framework has an advantage of having nano-sized pores and thereby providing a high surface area, and therefore is mainly used in adsorbing materials or immersing compositions in the pores and transferring the composition.
Relating to this, types, preparation methods and the like of a metal-organic framework having selective adsorptivity for oxygen, moisture or harmful substances (J. Am. Chem. Soc., 133(37): 14814-14822 (2011); Korean Patent No. 10-0806586; Korean Patent No. 10-0803945; Korean Patent Application Laid-Open Publication No. 10-0982641) have been reported, and an air treatment device including the metal-organic framework as an adsorbent (Korean Patent No. 10-1106840) has been reported. A French research group recently reported a research result that a metal-organic framework having an MIL-100 structure in which a chromium ion forms a skeleton has adsorption selectivity for carbon dioxide compared to methane, and therefore, PSA separation using the same may be accomplished (Dalton Trans., 41: 4052 (2012)). However, it was reported that the material used herein has a BET specific surface area of 1720 m2/g and a pore volume of 0.793 cm3/g, and this is 90% or less of a specific surface area and a pore volume of a material capable of being obtained under an optimal condition, and porosity declines since a significant amount of impurities remain in the pores. The concentration of a coordinatively unsaturated metal site is not sufficiently high during dehydration as well, therefore, it may be expected that the concentration of an adsorption site capable of selectively adsorbing relatively inert gas such as nitrogen is difficult to be secured. In addition, a theoretical result that a metal-organic framework having an MOF-74 structure prepared from a divalent vanadium ion and a 2,5-dioxido-1,4-benzenedicarboxylate ligand may be used as a nitrogen selective adsorbent was proposed (J. Am. Chem. Soc., 136: 698-704 (2014)), however, this is only a result proposed by computer simulation, and an actual experimental result capable of verifying the corresponding result was not able to be provided. Accordingly, a metal-organic framework having nitrogen selective adsorptivity has not been experimentally verified so far.